Answer

Export build-in Work Item "Planned For" attribute to CSV file first, and import the CSV file back to RTP afterword. In this case, you need to select "Internal Value".

During importing Work Items from CSV file back to RTC, iteration id is required to map to an existing iteration in Project Area. You can refer to the Importing work items from a CSV file topic in the Information Center for more details.

PURPOSE 2:

Export build-in Work Item "Planned For" attribute to CSV file for display. In this situation, you need to select "Label". This is used more frequently; however, there is still a chance to use "Internal Value" for this situation. For example, same iteration name has been used or you want use full iteration path.